Conventional powered ratchet wrenches, such as that disclosed in Japanese utility model gazette No. 1976-16,555, have a motor operated by compressed air in the base of the housing. When a throttle lever is pressed, compressed air flows to the motor, and the output shaft of the motor turns a transmission shaft by way of speed reducing gears. Slow speed and high torque are transmitted to the transmission shaft. The eccentric rotation of a crankshaft at the front end of the transmission shaft oscillates a ratchet yoke. The movement of the ratchet yoke causes the ratchet spindle or tool head of a ratchet mechanism to rotate so that a bolt, nut, or other threaded part is tightened or removed.
In a conventional ratchet wrench, the gear drive continues to transmit motor torque directly to the operator even after the fastener has been tightened to a specified tightening torque. That is, if the throttle lever is held open after the fastener has been firmly tightened, compressed air continues to drive the motor and the gears, which in turn drive the transmission shaft and ratchet mechanism. Thus, a considerable reaction force is transmitted to the operator as the tool tries to rotate around the tightened, stationary fastener. The operator's hand can be jerked forward by the wrench, or the operator may lose his grip. Even if the operator quickly releases the lever as soon as tightening is finished a reaction force is still transmitted to the hand. It is difficult to prevent the hand from being pulled along or from losing its grip. Hence, the operator usually releases the lever before tightening is finished. The operator then turns the tool manually to finish tightening. The tightening force applied by these prior art tools can therefore be inconsistent.
In some situations, conventional powered ratchet wrenches are unsuitable for use in tight places where there is room for only one hand. Because the ratchet wrench cannot be gripped tightly in such cramped places, and since it is difficult to release the throttle lever at exactly the right time, the hand is often jerked or loses its grip. The operator's hand can be forcefully thrown against an obstruction and injured, or the ratchet wrench can forcefully strike a projecting part and be damaged.
There is therefore a need for a powered ratchet wrench which minimizes the motor torque reaction force transmitted to the operator. It is desirable to provide a powered ratchet wrench which minimizes the torque reaction force transmitted to the operator's hand so that the hand is not pulled along with the tool while the motor is still operating and torque is still acting on the fastener.